THE SKELETON AND TEETH the whole life period. The result is that become increasingly prognathous, and the ridges, whic give origin to the temporal muscles, gradually increase in size ; in the Gorilla the enlargement of the crests reaches a maximum, Sexual differences* are seen in the whole build of the skull, the size of the jaws and the degree of development of crests for the muscles of mastication. The female cranial cavity is always smaller than that of the male, the difference being least in the Gibbons and greatest in the Gorilla. The actual measurements are given later. TO -L THE GIBBONS. Norma Verticalis.—The skull is ovoid in contour, and no prominent crests are present. But a well-marked line on each side marks the upper limit of the temporal muscle. The frontal bone may extend backwards to articulate with the occipital, but it usually articulates with the parietals by a curved coronal suture. The coronal and sagittal sutures usuallv meet; but the O V ' confluence is sometimes prevented by a lozenge-shaped Wormian bone. The vertex meets the plane of the occipital bone, which slopes downwards and forwards. Nonna Frontalis.—The orbits are capacious, and their borders are prominent. The upper margins form * The age and sex differences in the skull have been described by Bischoff and others. f Accounts of the skull in whole or in part have been written by Albrecht(4), Anderson (5), Bischoff (177), Duvernoy (53), Fry (188), Giebel (ipoX Hainy U51)* Huxley (87), Keith 1197), Kegnault 1380), Sehlegel (282), Seydel ('288), Wakleyei- <2i8), Yrolik (2171, de Blainville (20) and Deniker (44).